1. Field of the Invention
The invention relates to a gas sensor controller, and particularly to a gas sensor controller for obtaining current outputs and voltage outputs from a gas sensor provided with a heater.
2. Description of the Related Art
In general, the exhaust system of an internal combustion engine incorporates various gas sensors including an air-fuel ratio sensor (will hereinafter be referred to as “A/F sensor”). Such gas sensors are normally provided with a heater that is used to heat and thus activate the sensor element of the gas sensor promptly so that the gas sensor becomes able to produce outputs as intended as soon as possible. However, if the heated sensor element is moistened by condensed water, or the like, the temperature of the moistened portion of the sensor element sharply drops, which may cause cracking of the sensor element. Thus, various technologies for preventing such cracking of sensor elements due to moistening have been proposed (Refer to Japanese patent application publications No. 2003-83152 (JP-A-2003-83152) and No. 2001-41923 (JP-A-2001-41923). Japanese patent application publication No. 2003-83152 recites energizing the heater of the gas sensor such that the temperature of the heater is maintained lower than normal when it has been determined that moisture content is adhering on the inner face of the exhaust passage. That is, this technology is aimed at preventing cracking of the sensor element due to moistening.
Generally, in order to ensure that an A/F sensor produces accurate current outputs corresponding to the oxygen concentration in exhaust gas, the temperature of the sensor element of the A/F sensor needs to be maintained at approximately 750° C. Meanwhile, in order to determine at least whether the air-fuel ratio of exhaust gas is fuel-rich or fuel-lean based on the current outputs of an A/F sensor, the temperature of the sensor element of the A/F sensor needs to be maintained at approximately 500 to 600° C. at lowest. Therefore, in order to execute an air-fuel ratio control properly using the current outputs of an A/F sensor, the temperature of the sensor element of the A/F sensor needs to be maintained at approximately 500 to 600° C. at lowest.
Meanwhile, cracking of the sensor element of an A/F sensor can be prevented by maintaining the temperature of the sensor element at a level at and below which cracking of the sensor element does not occur (e.g., 300° C.) by limiting the amount of power supplied to the heater. In this case, however, although cracking of the sensor element can be prevented, the current outputs of the A/F sensor can not be used for the air-fuel ratio control, that is, it is impossible to enable early reduction of exhaust emissions. On the contrary, when the temperature of the sensor element of the A/F sensor is maintained at approximately 500 to 600° C. by controlling the energization of the heater, the temperature of the sensor element largely exceeds the level at and below which cracking of the sensor element does not occur even though the air-fuel ratio control can be executed at an early stage to reduce exhaust emissions. That is, if the heater is energized so as to enable early reduction of exhaust emissions, it inevitably creates a state where cracking of the sensor element due to moistening may occur.